1. Field of the Invention
The present invention relates to an inkjet recording apparatus, a method for inkjet recording, and an ink.
2. Description of the Related Art
An inkjet recording method includes discharging droplets of ink from a fine nozzle head and recording characters or symbols on a surface of a recording medium such as a paper.
Examples of known inkjet recording methods include a method in which an electric signal is converted into a mechanical signal with a piezoelectric element and ink stored in a nozzle head portion is intermittently discharged to thereby record characters or symbols on a surface of a recording medium, and a method in which an ink stored in a nozzle head portion is rapidly heated in the vicinity of an ink discharge port so as to generate bubbles and volume expansion due to the bubbles permits the ink to intermittently discharge to thereby record characters or symbols on a surface of a recording medium.
As an ink used for inkjet recording, for example, an ink containing a water-soluble dye, and an aqueous pigment ink obtained by dispersing pigment particles in water are known. The aqueous pigment ink has recently widely used because of its excellent water resistance and lightfastness.
Examples of the aqueous pigment ink include those obtained by dispersing pigment particles in an aqueous dispersion medium with use of a dispersant such as a surfactant or polymer dispersant and those obtained by dispersing pigment particles of which surface is made to be hydrophilic by, for example, an oxidation reaction.
In the ink obtained by dispersing pigment particles in an aqueous dispersion medium with use of a dispersant such as a surfactant or polymer dispersant, the dispersant is merely attached (adsorbed) to a pigment particle surface and where a strong shear force is applied to the pigment particles when the ink is discharged out through a fine nozzle of a nozzle head, the dispersant attached (adsorbed) to the pigment particle surface may be desorbed, thereby decreasing dispersibility of the pigment particles and deteriorating discharging stability of the ink.
Further, the above-mentioned ink has not sufficient storage stability because the residual of the dispersant remains in the ink, and the dispersant does not sufficiently contribute to dispersion and is detached from the pigment particles to thereby increase viscosity of the ink.
On the other hand, the ink obtained by dispersing pigment particles of which surface is made to be hydrophilic has good storage ability. However, an image formed with the ink has not sufficient fixability, abrasion resistance, and marker resistance against a highlighter pen.
For the purpose of enhancing fixability on a recording medium and dispersion (storage) stability of pigment particles contained in a pigment-type ink, an ink containing microencapsulated pigments obtained by coating colorant particles with a polymer is known.
However, the ink containing microencapsulated pigments results in decreased image density because pigment particles are coated with resin, which makes difficult to obtain a recorded matter having high print density.
An ink discharging head of an inkjet recording apparatus is coated with a water-repellent film in order to stably discharge ink droplets. When the water-repellent film is scraped off, water-repellency is decreased and the ink tends to easily attach and remain in the vicinity of an ink discharge port of a nozzle, which may deteriorate discharge stability.
Japanese Patent Application Laid-Open (JP-A) No. 2004-59913 discloses the ink in which the number of coarse particles is decreased to a certain number or less for the purpose of ensuring storage stability and discharge stability. However, an ink discharging head should be wiped with, for example, rubber blade periodically to remove an ink attached to the head. The wiping operation may scrape the water-repellent film off, which may reduce water-repellency. Accordingly, this ink also suffers from a problem of decreased discharge stability resulting from decreased water-repellency.